SUMMARY The development and widespread use of a dengue vaccine is required to significantly reduce the global dengue burden. A major goal of vaccine developers is to elicit robust immunity to each of the four antigenically distinct serotypes of dengue since all four serotypes can cause disease and death. It is likely that a zika virus vaccine will be available in the coming years. Pre-existing dengue virus immunity may impact the ability to generate an immunogenic zika vaccine. An improved understanding of the quality of Abs elicited during flavivirus infection is necessary as it is becoming clear that antibody function beyond neutralization is important. For this proposal, we will assess antibody dependent cell mediated cytotoxicity (ADCC) activity of antibodies present in well-defined zika immune sera (with and without previous dengue exposure). We have novel cell lines that enable us to assess the protein specificity of ADCC antibodies and also evaluate both target cell and effector cell activation. We will determine the relationship between antigen density and ADCC antibody titers in the sera. Finally, using pre-infection sera in individuals from Thailand who subsequently developed a subclinical or clinical dengue infection, we will correlate ADCC activity of antibodies with clinical outcome. The studies proposed are important to guide the development of an effective flavivirus vaccine.